1. Field of the Invention
The present invention relates to a pressure control apparatus for controlling the pressure at an output port by controlling a main valve through the switching operation of a directional control valve.
2. Discussion of Related Art
FIG. 5 shows a conventional pressure control apparatus for controlling the pressure at an output port to either of two pressures, i.e. high and low pressures, by controlling a main valve and a discharge valve through the switching operation of a directional control valve [see Japanese Utility Model Application Public Disclosure (KOKAI) No. Hei 1-72612]. In FIG. 5, a directional control valve 140 has a small input port 141, a small output port 142 and a small discharge port 143. In a state where the small output port 142 is communicated with the small discharge port 143, a pressure fluid is supplied to a supply port P. A pressure regulating member 125 is in the position shown in the figure. The supplied pressure fluid opens a main valve element 114 to flow to an output port A and also passes through a feedback passage 120 to flow into a feedback chamber (diaphragm chamber) 119. Consequently, the fluid pressure in the feedback chamber 119 rises. When the pressure in the feedback chamber 119 reaches a first set pressure (a set low pressure), a diaphragm 118 moves to a neutral position, causing the main valve (114, 113) to be closed. When the fluid pressure at the output port A becomes lower than the first set pressure, the main valve (114, 113) opens to allow the pressure fluid to flow to the output port A. When the fluid pressure at the output port A becomes higher than the first set pressure, the main valve (114, 113) is closed, and a discharge valve (122, 121) opens. Consequently, the pressure fluid at the output port A is discharged. In this way, the fluid pressure at the output port A is controlled to the first set pressure.
Next, when the directional control valve 140 is switched to allow the small output port 142 to communicate with the small input port 141, the pressure fluid from the supply port P flows into a pressure setting chamber 127 through the small input port 141 and the small output port 142. Consequently, the fluid pressure in the pressure setting chamber 127 rises, causing the pressure regulating member 125 to move downward. The pressure regulating member 125 stops when it abuts against an engagement portion 136 of a high-pressure setting member 135. The downward movement of the pressure regulating member 125 compresses a pressure setting spring 128 and hence increases the urging force of the pressure setting spring 128 acting on the diaphragm 118. The increase in the urging force of the pressure setting spring 128 allows the fluid pressure at the output port A to be controlled to a second set pressure (a set high pressure).
With the prior art shown in FIG. 5, a pressure fluid at either of the first and second set pressures is obtained, and each set pressure is controlled to a constant pressure by feedback control. However, the feedback control is not always needed for some use applications of the fluid pressure cylinder. A first example of such use applications is as follows. During a stroke L1 of the full stroke L of the piston of the fluid pressure cylinder, the main valve is fully opened to move the piston at high speed, thereby shortening the stroke time. During the remaining stroke L2, the piston is moved at a set pressure to control pressing force (e.g. a caulking machine). Alternatively, during the stroke L1, the piston is activated at a set low pressure to ensure safety. During the remaining stroke L2, the main valve is fully opened to activate the piston at high pressure (e.g. an automatic door). In a second example, a fluid at a set pressure is supplied for a desired period of time. Thereafter, the flow of the fluid is stopped (e.g. an air blower or a fluid pressure motor). The prior art shown in FIG. 5 has functions more than is needed for the first and second examples. Therefore, application of the prior art to the first or second example is a waste of its capabilities.